This invention pertains to the art of refrigerant compressor oil filtering systems.
A typical transport refrigeration unit with which my invention is mainly intended to be used is generally shown in U.S. Pat. No. 4,419,866, for example. As may be there seen, the refrigeration system includes a considerable number of parts such as valves, condenser, evaporator, receiver, accumulator, heat exchanger, and so on, which are assembled to make the total refrigeration system to which the refrigerant is fed from the compressor. In the system some oil is always carried through the system along with the refrigerant. This oil-refrigerant mixture makes a good degreasing and cleaning agent which will pick up and return the compressor contaminants such as rust, scale, metallic particles and dirt from any components in the system which are not totally clean. Such contaminants may also be found in a newly manufactured compressor itself, depending upon the factory environmental conditions and cleanliness standards where the compressor is made.
Since these transport refrigeration units are relatively expensive machines, production testing of substantially all is done before turning units over the customers. In such a test, lasting about an hour and during which the mechanical and refrigeration functions of the unit are checked out, the oil-refrigerant mixture circulates throughout the system and, if some parts of the system are unduly dirty, the contaminants returned to and deposited in the compressor oil sump may require that the oil be replaced. Since the compressor is part of an overall refrigeration system which is hermetically sealed, this entails certain procedures to avoid loss or depletion of the refrigerant charge. In other words, it is not as simple as merely changing oil in, say, an air compressor or internal combustion engine where the other medium (air) is not a charge that can be lost or in effect dirtied.
Even if the oil is once replaced after initial unit testing, this does not insure that there might not still be sufficient contaminants in the system that after a certain amount of additional operation, the compressor will fail due to contaminants plugging the intake to the compressor oil pump so that the pressurized lubrication system cannot deliver sufficient oil to the bearings and seals of the compressor, or the fact that the pickup screen cannot be tight enough mesh to catch all small particles of contamination without seriously restricting the needed oil volume to sustain the compressor lubricating system. These contaminants will conceivably wear on surfaces to be lubricated; therefore prematurely reducing the serviceable life of these areas.
Most refrigerant compressors for transport refrigeration units are generally similar in overall construction and typically include a positive displacement oil pump at the end of the crank shaft opposite the driven end. The oil system also includes a screen type filter of one kind or another in the oil sump to prevent the passage or larger particle contaminants from passing through the screen to the oil pump. The screen (of, say, 40 mesh) is effective to prevent larger particles from passing to the pump. However, much smaller particles can build up onto larger particles and in agglomerating fashion upon the screen to build up a pasty mass which plugs the screen so that the oil reaching the pump is insufficient to produce sufficient flow to the pressurized lubricating system, or the small particles can pass through causing wear to all lubricating surfaces. This results in loss of the compressor which, in view of the possibility that the load carried in the trailer which the unit is serving may be readily perishable and have a value magnitudes larger than the value of the compressor itself, or even the value of the refrigeration unit as a whole, can be disastrously expensive to someone.
From the foregoing it can be seen that having oil which has a limited quantity of the relatively smaller particles can prevent compressor failure due to lubrication failure. In that connection there are significant differences between oil systems for refrigerant compressors and oil systems for internal combustion engines, for example. In internal combustion engines there is constant contamination being generated by the fact of combustion, etc. and from time to time the oil must be drained and replenished. In a refrigerant compressor, the oil is being carried along with the refrigerant through the entire closed refrigeration system. If the refrigeration system as a whole is adequately clean initially, and there is the proper oil level in the compressor, the oil need never be changed.
Whether the oil in a compressor is too dirty or not due to small particles cannot be accurately determined by appearance since the oil color may be deceptive. An instrument such as a ratio turbidimeter, which uses light refraction to measure the amount of suspended solids in a liquid, can be used as an accurate determination. It has been concluded that if oil has an NTU (nephelometric turbidity units) value of less than, say, 90 NTUs, this oil will be satisfactory for extended use of compressor and, perhaps, for the life of the compressor so long as there is no subsequent internal contamination such as can occur through any opening of the refrigerant system.
The aim of this invention is to provide a bypass oil filtering system particularly adapted to filter oil for a refrigerant compressor of the type used with a transport refrigeration unit.